Stationary UPS Sizing Calculations – Part Three

As we stated in the previous article “Stationary UPS Sizing Calculations -Part One” That Stationary UPS Sizing Calculations include:

  1. The UPS sizing calculations,
  2. Rectifier/Charger sizing calculations,
  3. Inverter & Static Switch sizing calculations,
  4. The Battery sizing calculations.

We explained the UPS sizing calculations in the above article and we explained in article “Stationary UPS Sizing Calculations -Part Two, the following calculations:

- Rectifier/Charger sizing calculations,

- Inverter & Static Switch sizing calculations,

- The Battery sizing calculations, which includes:

First: The Manufacturers’ methods, which include:

  • Method#1: Watts per cell method,
  • Method#2: Watts per bank method,
  • Method#3: Ampere per cell method.

 

Today, we will explain the Second methods of Battery sizing calculations; The IEEE methods.

 

 

 

Second: The IEEE Methods of Battery Sizing Calculations

 

  

 

The IEEE methods include:

  • Method#1: The IEEE 485 method,
  • Method#2: The IEEE 1184 method.

 

  

 

Method#1: IEEE Standard 1184 Batteries for Uninterruptible Power Systems

 

  

 

This method is intended to assist engineers who select and design battery systems for uninterruptible power systems (UPS) from the two main battery types:

  1. Lead-acid batteries, which include both vented (flooded) and valve-regulated (sealed) constructions
  2. Nickel-cadmium batteries

 

  

 

Important Terminology

 

1- Equalizing charge - Nickel-cadmium cells

Recommended equalizing voltages range from 1.47-1.65 V/cell. To achieve reasonable end-of-discharge voltages, the equalizing voltage attainable in a specific application is normally restricted by system voltage limitations to about 1.55 V/cell.

Generally, equalizing is used for a fast recharge after a discharge, and to assist in electrolyte mixing after water addition (two rate chargers are generally recommended by the manufacturers). Depending on the float voltage chosen, routine periodic equalizing may be unnecessary.

 

2- Temperature – Nickel cadmium

All batteries are affected by high temperatures. However, nickel-cadmium cells can sustain high temperatures more easily than most other systems because the chemistry in the active materials is relatively stable.

For example, at 32 °C (90 °F) the normal life of a nickel-cadmium cell is diminished by about 20%, compared with a reduction of approximately 50% for a lead-acid cell.

Moreover, a nickel-cadmium battery will not be destroyed by low temperatures or freezing. With a normal electrolyte, the battery will operate at temperatures as low as 30 °C (86 °F) to 40 °C (104 °F); with a higher specific gravity electrolyte, it will operate at even lower temperatures. The available capacity is reduced at low temperatures, but at 40 °C (104 °F) a nickel-cadmium battery can deliver 60% or more of rated capacity.

 

3- Temperature - Lead acid

Operating batteries at temperatures below 25 °C (77 °F) decreases performance. Note that temperature derating factors (see IEEE Std 485-1983) apply to the discharge rate and not the discharge time.

For example, a temperature derating factor of 1.11 for a certain cell type at 15.6 °C (60 °F) indicates that battery performance is approximately 10% less than at 25 °C (77 °F). If this battery can supply, say, 100 kW for 15 min at 25 °C (77 °F), it will be capable of delivering 90 kW for 15 min at 15.6 °C (60 °F). Conversely, the battery may be able to supply the same 100 kW load for only 10 min at 15.6 °C (60 °F), because of the decrease in battery efficiency at higher discharge rates. The extent of this effect varies with cell construction.

 

  

 

The design steps for The IEEE 485 method

  

Step#1: calculate Battery Total load

Step#2: calculate the corrected Battery Total load

Step#3: calculate the maximum number of cells

Step#4: calculate the end voltage

Step#5: calculate the Minimum number of cells

Step#6: Cell selection

 

Note:

For each battery type, the design steps are described in order to facilitate the selection. Noting that each design has advantages and disadvantages, all of which should be considered when selecting a battery.

 

 

 

 

Case#1: for Lead-acid battery calculations

 

  

 

Initial assumptions and limiting factors

Battery discharge voltage range: 1.67- 2.10 V/cell

Battery equalizing charge range: 2.30- 2.50 V/cell

 

Step#1: calculate Battery Total load

 

Battery Total load (W) = UPS Load (W) / inverter efficiency

Or

Battery Total load (W) = UPS Load (W) + inverter losses (W)

 

Example#1:

System size: 500 kVA at 0.80 PF, 400 kW

System output voltage: 3 ph, 120/208 Vac (not required for calculation)

Inverter efficiency: 0.92 efficiency at full load (dc input to ac output)

 

Solution:

Battery Total load (W) = 400 kW/0.92 = 435 KW

 

Note:

The very latest generation of on-line UPS have inverter efficiencies of up to 97%, producing longer battery autonomies than could previously be achieved from the same battery connected to a UPS with a less efficient inverter.

 

 

 

 

Step#2: calculate the corrected Battery Total load

 

Corrected Battery Total load = Battery Total load × ageing factor × temperature correction factor x design Margin

 

Where:

- Ageing factor

Since the battery capacity does not remain at its nameplate rating throughout its life, a 25% margin will be included as an aging factor for the 435 kW.

 

- Temperature correction factor

Also, since it is expected that the operating temperature will drop to a low of 15.6 °C (60°F), the battery capacity should be increased by another 11% (per IEEE Std 485-1983) to ensure that it will provide rated load at reduced temperature.

 

- Design Margin

A design margin is taken into account for any inaccuracies in the load’s estimation. Generally, a design margin ranges between 10% and 15% is suggested. And sometimes it will be ignored.

 

Then, in continued example#1:

Corrected Battery Total load = 435 KW * 1.25 *1.11 = 604 KW

 

 

 

Step#3: calculate the maximum number of cells

 

In Continued example#1:

It is assumed that the manufacturer recommends an equalizing voltage of 2.40/cell.

 

The maximum number of cells = Maximum system voltage / Recommended equalizing V/cell

Then,

The maximum number of cells = 432/2.40 = 180 cells

 

Note:

In this case no adjustment is made for the voltage losses in the cables and cell connectors. In the final stages of battery recharge the current drops to a point where the voltage drops are insignificant on the large cables that have been sized for the final discharge currents.

 

  

 

Step#4: calculate the end voltage

 

Note:

In order to take full advantage of the battery’s usable capacity, the lowest possible end-of-discharge cell voltage should be used. This is subject to the limits imposed by the minimum allowable system voltage and the battery manufacturer’s stated minimum cell voltage for the discharge time in question.

More important, the battery must first be capable of being charged in accordance with the manufacturer’s recommendations and within the maximum system voltage limit.

 

The final voltage per cell = Minimum battery voltage/Number of cells

 

In Continued example#1:

Allowing for the planned 2 V loss in the cables, the minimum battery voltage of 292 V is then used to calculate the final voltage per cell:

The final voltage per cell = Minimum battery voltage/Number of cells = 292/180 = 1.62 V /cell

 

  

 

Step#5: calculate the Minimum number of cells

 

Note:

In most cases, the calculated number of cells and minimum voltage per cell would be used directly in the remainder of the battery sizing exercise. However, in this example, it is assumed that the battery manufacturer states a minimum discharge voltage of 1.67 V/cell for a 20 min discharge. Since the calculated minimum voltage is 1.62 V/cell and the manufacturer’s minimum voltage is 1.67 V/cell, it is necessary to adjust the number of cells to reflect the higher value of the minimum discharge voltage.

 

Minimum number of cells = Minimum battery voltage/ Minimum discharge V/cell

 

Therefore, In Continued example#1:

Minimum number of cells = Minimum battery voltage/ Minimum discharge V/cell = 292/1.67 = 175 cells

 

  

 

Step#6: Cell selection

 

At this point it has been determined that the battery required is one with 175 cells that can deliver 604 kW for 20 min and not drop below 1.67 V/cell. Each cell shall then deliver:

604 kW/175 cells = 3.45 k W/cell

 

Now there is complete information with which to consult the manufacturer’s performance charts and select the proper cell for the application (20 min, 1.67 V/cell, and 3.45 kW/cell).

 

It may be beneficial to repeat the calculation to optimize the number of cells for a particular cell type. For example, if there is a cell that can provide 3.40 kW/cell, it would probably be more economical to increase the number of cells, rather than using 175 of the next larger cell size. In this case, the new number of cells would be:

 

604 kW/3.40 = 178 c ells

 

Notes:

- Changing the number of cells will affect both:

  1. The equalizing Voltage and
  2. The end-of-discharge voltages.

 

- Increasing the number of cells allows a lower end-of-discharge voltage per cell (more usable capacity) within the lower system limit, but may result in a required equalizing voltage that is higher than the upper system limit.

- Decreasing the number of cells will not impose any constraints on the maximum voltage limit, but will result in a higher end-of-discharge voltage per cell (less available capacity).

- In this particular example, it is already known that 180 cells can be accommodated within the upper system voltage limit. At the lower voltage limit, the use of 178 cells would allow discharging to 1.64 V/cell, but would fail to meet the battery manufacturer’s stated minimum of 1.67 V/cell.

- Battery selection, then, is a process of finding the best fit between the maximum charge voltage and the minimum operating point of the UPS that will allow the maximum use of the available battery capacity.

 

 

 

 

Case#2: for Nickel-cadmium battery calculations

 

  

 

Initial assumptions and limiting factors

Battery equalizing voltage 1.52 V/cell

Battery discharge voltage 1.00 - 1.10 V/cell

 

Step#1: calculate Battery Total load

As in example#1

 

 

 

Step#2: calculate the corrected Battery Total load

 

Since the design life (assumed to be 15 years) is well within the life expectancy of vented nickel-cadmium batteries, a 10% margin will be included as an aging factor for the 435 kW.

Typically, temperature factors are used in nickel-cadmium battery sizing only when there is a considerable deviation from room temperature. In this example, the 15.6 °C (60°F) minimum temperature typically would give rise to a capacity reduction of only 3% or so. This figure would generally be incorporated into the overall battery sizing margins to give a combined figure for both aging and low temperature operation.

 

Then,

Corrected Battery Total load = Battery Total load × ageing factor × temperature correction factor x design Margin

 

 Corrected Battery Total load = 435 kW x 1.10 = 479 kW

 

  

 

Step#3: calculate the maximum number of cells

 

Selection of number of cells (cell type) and end voltage point

The most economical battery choice results from using the lowest end-of-discharge voltage and the largest possible number of cells that will satisfy the manufacturer’s recommendations. The first step in the battery calculation is to ensure that the battery can be properly charged.

For this example, it is assumed that the manufacturer recommends a minimum equalizing voltage of 1.52 V/cell. The maximum number of cells is therefore:

 

The maximum number of cells = Maximum system voltage / Recommended equalizing V/cell

 Then,

The maximum number of cells = 432/1.52 = 284 cells

 

  

 

Step#4: calculate the end voltage

 

Note:

In this case no adjustment is made for voltage losses in the battery cables and cell connectors. In the final stages of battery recharge the current drops to a point where the voltage drops are insignificant on the large cables that have been sized for the final discharge currents.

 

Allowing for the planned 2 V loss in the cables, the minimum battery voltage of 292 V is then used to calculate the final voltage per cell:

 

The final voltage per cell = Minimum battery voltage/Number of cells

 

 Then,

The final voltage per cell = 292/284 = 1.03 V/cell

 

  

 

Step#5: calculate the Minimum number of cells

 

 

Minimum number of cells = Minimum battery voltage/ Minimum discharge V/cell

 

Then,

Minimum number of cells = 292/1.1 = 266 cells

 

  

 

Step#6: Cell selection

 

At this point it has been determined that the battery required is one with 284 cells that can deliver 479 kW for 20 min and not drop below 1.03 V/cell.

 

Minimum discharge V/cell = 479/284 = 1.69 k W/cell

Now there is complete information with which to consult the manufacturer’s performance charts and select the proper cell for the application. When using tabular data, it may be necessary to interpolate between published values.

For instance, in the example the cell type required is one that will supply 1.69 kW/cell for20 min to a final voltage of 1.03 V/cell. In this case it would probably be necessary to interpolate between values given for a final voltage of 1.05 V/cell and 1.00 V/cell. Since most manufacturers offer high, medium, and low rate cell ranges, it may be advisable to determine cell sizes for two or more of the ranges. The most economical option meeting the above parameters can then be chosen. Battery selection then, is a process of finding the best fit between the maximum charge voltage and the minimum operating point of the UPS that will allow the maximum use of the available battery capacity.

 

  

 

 

Method#2: IEEE 485 Lead Acid Batteries for Stationary Applications

 

 

 

 

This standard details methods for defining the dc loads and for sizing a lead-acid battery to supply those loads in full float operation

The IEEE 485 design method has the five steps as follows:

 

Step#1: develop the overall connected load list that the battery requires to supply

Step#2: Develop a load profile

Step#3: Calculating the Design Load & Energy Demand Design Load

Step#4: Choose the type of battery and determine the cell characteristics

Step#5: On the basis of design loads, calculate the desired Ampere-hour (Ah) battery capacity

 

  

 

Step#1: develop the overall connected load list that the battery requires to supply

 

The first step is the determination of the total connected loads that the battery needs to supply and making a load list. This is mostly particular to the battery application like UPS system or solar PV system.

 

For this purpose, Loads are classified as:

  • Continuous - loads continually present
  • Non-continuous - loads lasting for a specific period
  • Momentary - loads lasting for less than 1 minute

 


Calculating the Consumed Load ( VA )

Here, we can calculate the consumed apparent power of the loads in terms of Volt-Ampere. For every single load, we can compute the VA using the following formula:


S=Pcosϕ×η

Where

S, load consumed apparent power (VA)

P, load consumed active power (W)

η, load efficiency (pu)

cosϕ, load power factor (pu)

 

  

 

Step#2: Develop a load profile

 

The load profile is developed from the load list that demonstrates the load’s distribution over the period of time.

The standard recommends a duty cycle be drawn showing the anticipated loads (in Ampere or power) for the required duration of battery backup time and take in Consideration the following:

  • Loads and times where known should be shown
  • Random loads should be shown at the most critical times

 


Figure-1: Duty Cycle Diagram

There are two various methods to develop load profiles:

  1. 24 Hour Method
  2. Autonomy Method

 

Note:

Both methods use the same first three general steps with some minor differences.

 

1- the 24 Hour Method

This method exhibits the average instantaneous loads over a period of 24 hours and it is primarily utilized in solar PV system applications.

In the 24 hour profile method, the associated time period is represented in tens of "ON" and "OFF" times. These are the times in the day (in hours and minutes) that the load is expected to be switched on and then later turned off. For loads that operate continuously, the ON and OFF time would be 0:00 and 23:59 respectively. A load item may need to be entered in twice if it is expected to start and stop more than once a day.

 

 

2- the Autonomy Method

This method is primarily utilized in backup power applications such as battery systems in UPS. This method exhibits the average instantaneous loads over an autonomy (backup) time, which is the time period (the number of hours) in which the load needs to be supported during a power supply interruption

The “Autonomy method” for constructing load profile is typically used for AC UPS system & batteries. And we get it either from:

 

  • The autonomy time is often specified by the client ( in their standards )
  • Or IEEE 446 “IEEE Recommended Practice for Emergency and Standby Power System for Industrial and Commercial Application” has some guidance for autonomy time.

 

Sometimes a single autonomy time is used for entire UPS load, which obviously makes the construction of the load profile easier to compute.

Some loads may only be required to ride through brief interruptions or have enough autonomy to shut down safely, while some critical systems may need to operate for as long as possible (up to several days)

 

How to develop he load profile using Autonomy Method?

 

The load profile in the Autonomy Method is developed by heaping “energy rectangles” on top of one another. In this energy rectangle, height represents the load (VA) and the width represents the autonomy time (backup time) whereas the rectangle area represents the total load’s energy. The load profile is produced by heaping the broadest rectangles first.

 

 

Example#2:

Let’s assume the loads given in the following table are based on the Autonomy (Backup) Method:

 



For example of drawing the load profile for every load, the Digital Cross-Connect Section will be a rectangle of width 4 (hours) and height of 200 (VA). The load profile is produced by heaping the broadest rectangles first so the telecom section will be the first represented load

 

 


 

Figure-2: Load Profile for the Battery Sizing Example

 

 

 

 

 

Step#3: calculating the Design Load & Energy Demand Design Load

 

First: the Design Load

The design load is the one for which all the system devices should be rated such as fuses, breakers, cables, inverters, rectifiers. The design load can be computed using the following equation:

 

Sdes=Speak(1+kcont)(1+kdm)

Where:

Sdes, design load in VA

Speak, peak load in VA

kcont, contingency factor for load growth (%)

kdm, design margin (%)

 


Notes:

  • It is normal to take into account future load growth in the calculations which generally ranges between 5 % and 20 % (typically somewhere between 5 and 20%).
  • A design margin is taken into account for any inaccuracies in the load’s estimation. Generally, a design margin ranges between 10% and 15% is suggested.

 


Example#3:

 Let’s assume that the peak load apparent power is 640 VA. Considering a future growth of 10 % and a design margin 10%, the total design load is:

 

Sdes=Speak(1+kcont)(1+kdm)=640×(1+0.1)(1+0.1)=774.4VA


 

Second: the Energy Demand

The design energy requirement (VAh) is utilized for energy storage devices sizing. The total design energy can be calculated by computing the area under the load profile curve. The total design energy requirement can be computed using the following equation:

 

Ede=Etle(1+kcont)(1+kdm)

Where:

Ede, total design energy required (VAh)

Etle, total load energy (area under the load profile) in VAh

kcont, contingency factor for load growth (%)

kdm, design margin (%)

 


Example#2 -continued:

From table 1 above, the total load energy is 2,680VAh. Considering a future growth of 10 % and a design margin 10%, the total design energy required is:

 

Ede=2680×(1+0.1)(1+0.1)=3243VAh

 

  

 

Step#4: Choose the type of battery and determine the cell characteristics

 

The following step is the selection of the type of battery (e.g. Lead-acid or nickel-cadmium). While choosing the battery type, the following elements should be considered as per IEEE guidance:

 

  • Ambient temperature threshold
  • Charging & discharging characteristics
  • Maintenance & Ventilation requisites
  • Cell orientation essentials
  • Shock and vibration factors
  • Anticipated cell life
  • Physical properties like dimensions, weight, and battery terminals

 

Next is to determine the battery cell characteristics which are generally provided in manufacturer’s data sheet. The primary cell characteristics that should be considered are:

 

  • Ampere-Hour capacities of battery cell
  • Temperature of battery cell
  • Electrolyte density in case of lead-acid batteries at a full charge
  • Cell float voltage of cell
  • Cell end-of-discharge voltage (EODV) of cell

 

Battery’s Ampere-Hour capacities are provided by the battery manufacturer on the basis of various EODVs. For lead-acid type batteries, an EODV is principally based on an EODV value that prohibits cell damage by over-discharge. Generally, EODV ranging between 1.750V and 1.80Vis utilized per cell when discharging time is longer than 1 hour. For short discharging time (<15 minutes), an EODV of about 1.66V per cell may be utilized without cell damage.

 

 

Total Number of Cells

The number of cells required for a particular voltage rating is presented below:

 



But, the number of cells required can be determined more precisely in order to match with the load tolerance more accurately. The number of battery cells expected to be linked in series fashion must fall between the two limits (Nmin & Nmax) which are given below:

 

The number of cells in a battery is computed to match the minimum and maximum voltage tolerances of the load. As a minimum, the battery at its EODV must be within the minimum voltage range of the load. Meanwhile, as a maximum, the battery at float voltage (or boost voltage) needs to be within the maximum voltage range of the load. The cell charging voltage depends on the type of charge cycle that is being used, e.g. float, boost, equalizing, etc, and the maximum value should be chosen.

The number of battery cells required to be connected in series must fall between the two following limits:

 

Nmin = Vdc*(1+Vmax) / Vc

Nmax = Vdc*(1-Vmin) / Veod

 

Where:

 

Nmin = minimum number of battery cells

Nmax = maximum number of battery cells

Vdc = nominal battery voltage (V)

Vmin = minimum load voltage tolerance (%)

Vmax = maximum load voltage tolerance (%)

Veod = cell end of discharge voltage (V)

Vc = cell charging voltage (V)

 

Choose the required number of cells within these two limits (although choosing cell numbers in the middle of minimum and maximum values would be most suited).

 

Example#3:

The minimum and maximum load voltage tolerances are Vmin = 10% and Vmax = 10%, respectively. For the battery, nominal voltage is Vdc = 120V, end-of-discharge voltage is Veod = 1.8V/cell, and the cell charging voltage is Vc = 2.05V/cell.

Then,

 

Nmax = 120(1+0.1) / 2.05

Nmax = 64.4 or 64 cells

 

Nmin = 120(1-0.1) / 1.8

Nmin = 60 cells

 

Use 62 cells in series (number of cells between 60 and 64).

 

Another method for calculating Number of Cells and Cell Voltage

 

The number of cells is estimated based on the maximum battery voltage and float charge voltage:

 

Number of cells = maximum voltage / float charge voltage

 

The minimum battery voltage is the minimum system voltage (including voltage drops across cables).  Given the minimum cell voltage the minimum cell voltage is given by:

Minimum cell voltage (V/cell) = minimum battery voltage / number of cells

 

  

 

Step#5: calculate the Desired Ampere-Hour (Ah) Battery Capacity

 

The battery capacity desired to accommodate the total designed load over the determined back up (autonomy) time can be calculated using the following formula:

 

Cminimum= Ede(kaf×ktcf×kcrt) /( Vdc×kmdod×kse)

 

Where:

Ede, total designed energy over back up time (VAh)

kaf, Battery Aging Factor (%)

ktcf, Temperature Correction Factor (%)

kcrt, Capacity Rating Factor (%)

Vdc, Battery Voltage (Nominal)

kmdod, Maximum depth of Discharge (%)

kse, System Efficiency (%)

 

Choose a battery capacity (Ampere-Hour) that surpasses the minimum capacity calculated using the above battery sizing formula.

 

An explanation of the various elements:

 

- Aging Factor:

It actually represents the reduction in battery performance with the passage of time. To ensure the battery can meet the design requirements throughout its life the standard suggestions the initial capacity should be 125% of the design capacity.

 

- Temperature correction factor:

The battery cells capacity is generally provided for a standardized temperature which is 25oC and if it varies, the battery cells capacity will vary as follows:

 

Temperature decreases, the battery cells capacity decreases

And vise verse as the temperature increases, the battery cells capacity increases

 

So, a correction factor is needed to implement and this correction factor we can get it from the below table.


 

Temperature correction factor Table

 

- Capacity rating factor

This factor accounts for voltage reduction during the discharge of the battery. In Lead-acid batteries, a voltage dip occurs in the early phases of battery discharge followed by certain recovery.

 

There are two ways of expressing capacity:

Term Rt

The term Rt is the number of amperes each plate can supply for t minutes, at 25oC to a defined minimum cell voltage.

Ct = Rt

 

Term Kt

The term Kt is the ratio of ampere-hour capacity, at a standard time rate, at 25oC and to a defined minimum voltage which can be delivered for t minutes.

Ct = 1/Kt

 

Rt is not equal to 1/Kt because each factor is expressed in different units.

 

- System efficiency

It accounts for battery losses (coulombic efficiency) as well as power electronics losses (such as charger and inverter).

 

- Design Margin 

It accounts for unexpected circumstances (increased loads, poor maintenance, recent discharge, etc.) it is common to allow a design margin of 10% to 15%.

 

- Depth of discharge (DOD)

Consider shallow DOD (max 20% recommended) and occasional deeper DOD (max 80%)

 

Example#4:

calculate for the minimum battery capacity using the given energy demand of 4020 VAh, battery aging factor (kaf) = 25%, temperature correction factor at 30 deg C (ktcf) = 0.956, capacity rating factor (kcrt) = 10% and depth of discharge (kmdod) = 80%, system efficiency (kse) = 95%

 

Cminimum = Ede(kaf×ktcf×kcrt) /( Vdc×kmdod×kse)

 

Cminimum = [4020 VAh X (1.25 X 0.956 X 1.1)] / (120 V X 0.8 X0.95)

Cminimum = 57.94 Ah

Use a VRLA battery with a 60 Ah capacity.

 

  

 

Example#5: Battery Sizing Calculation

 

Steps#1 and #2: Collect All the Connected Loads and Develop a Load Profile

In this particular example, we will apply the same loads and load curve provided in the Load Profile Calculation Example. The load profile for this case is demonstrated in the figure below.




 

Step#3: calculating the Design Load & Energy Demand Design Load

 

From the load profile, the following parameters were computed:

Total Design Energy Demand = Ede = 3,245 Vah

 

Step#4: Choose the type of battery and determine the cell characteristics

For this particular example, a vented lead-acid battery has been chosen.

 

We assumed the following values in order to calculate number of cells required:

Vdc=120V

Vload,min=10

Vload,max=20

Veodv=1.80V/cell

Vcharging=2.25V/cell

 

Maximum number of cells required to be connected in series:

Nmaximum=Vdc(1+Vload,max)/Vcharging=120×(1+0.2)/2.25=64 Cells

 

Minimum number of cells required to be connected in series:

Nminimum=Vdc(1−Vload,min)/Veodv=120×(1−0.1)/1.80=60 Cells

 

The number of cells chosen for this example is 62 cells which is in between the maximum and minimum limits.

 

Step#5: calculate the Desired Ampere-Hour (Ah) Battery Capacity

We assumed the following values in order to compute the battery capacity:

Ede=3245 VAh

kaf=0.30

ktcf=0.96

kcrt=0.12

Vdc, Battery Voltage (Nominal)

kmdod=0.75

kse = 1

Using the above mentioned parameters, we can compute the minimum battery capacity as:


Cminimum=Ede(kaf×ktcf×kcrt)/Vdc×kmdod×kse

Cminimum=3245×(1.30×0.96×1.12)/(120×0.75x1)=50.4 Ah

 

Choose a battery capacity (Ampere-Hour) that surpasses the minimum capacity computed using the above formula.

 

 

  

 

Very important Note

 

Mixing different battery sizes or types in a system is generally not recommended due to variations in voltage, capacity, and charging/discharging characteristics. It is best to use batteries of the same type, capacity, and age to maintain optimal performance and balance within the system.

 

 

In the next Article, we will explain the following:

  • UPS Backup time calculation
  • Selection and sizing of UPS protective devices (CBs or Fuses)
  • Selection and sizing of UPS Cables
  • Sizing a generator set for UPS system
  • UPS room ventilation calculation

 

So, please keep following.

 

 

Subject Of Pervious Article

Article

Applicable Standards for UPS Systems

  • What is a UPS?
  • Why do we need a UPS?
  • UPS Rating
  • Classification of UPS:

1-Voltage range,

2-No. of phases,

3- Mobility,

4- Technological design,

 

Classification and Types of UPS – Part One


5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

Classification and Types of UPS – Part Two


6.2- “N+1” System Configuration, which includes:

  • Isolated Redundant Configuration (N +1)
  • Parallel Redundant Configuration (1+1)
  • Parallel Redundant Configuration (N +1)
  • Parallel Redundant Configuration (N +2) and so on

6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

 

Classification and Types of UPS – Part Three


 

6.4- Parallel Redundant with STS Configuration

  • Parallel Redundant Configuration (1+1) + STS
  • Parallel Redundant Configuration (N+1) + STS

6.5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N

 

Classification and Types of UPS – Part Four


 

 

7- According to UPS Topology

7.1 Off-line or Standby UPS,

7.2 Line Interactive UPS,

7.3 Standby-Ferro UPS,

7.4 Online Double Conversion UPS,

7.5 The Delta Conversion On-Line UPS.

Classification and Types of UPS – Part Five

 

 

 

8- According to UPS Distribution Architecture

8.1 Centralized UPS Configuration,

8.2 Distributed (Decentralized) UPS Configuration,

8.2.1 Distributed UPS-Zonewise Configuration

8.3 Hybrid UPS Configuration.

Conventional (Monolithic) Vs Modular UPS System:

  • Deploy UPSs in parallel,
  • Deploy UPSs in Series,
  • Use modular UPS products.

Classification and Types of UPS – Part Six


 

Three Basic Configurations Of Mains And Bypass For A UPS System:

  • Single mains,
  • Single mains without bypass,
  • Dual mains.

9-According to Use of transformers with the UPS

  • Transformer based,
  • Transformer less UPS,
  • Transformer less UPS with external input/ output transformer.

 

Classification and Types of UPS – Part Seven


 

 

Transformer Arrangements in Practical UPS Systems:

1-Transformer options for the “single mains” configuration

2-Transformer Options for the “Dual Mains” Configuration

Classification and Types of UPS – Part Eight


 

3- Transformer options for “single mains without bypass”

Classification and Types of UPS – Part Nine

Components of Online Double Conversion UPS:

1- Rectifier,

2- Inverter,

3- Energy Storage system:

3.1 Battery

Components of Online Double Conversion UPS– Part One


 

 

3.1.1 Battery Configurations

  • Serial Strings,
  • Parallel Strings.

3.1.2 Battery Size and Location

3.1.3 Battery Transition Boxes

3.1.4 Battery Monitoring

3.2 Energy Storage System – Flywheel

3.3 Energy Storage system – Super Capacitors

3.4 Hydrogen Fuel Cells

4- Static switch

Earthing Principles of UPS Systems

Components of Online Double Conversion UPS – Part Two


 

Evaluation Criteria for Selecting an UPS:

Step#1: Determining the need for an UPS,

Step#2: Determining the purpose(s) of the UPS,

Step#3: Determining the power requirements,

Step#4: Selecting the type of UPS,

Step#5: Determining if the safety of the selected UPS is acceptable,

Step#6: Determining if the availability of the selected UPS is acceptable,

Step#7: Determining if the selected UPS is maintainable, and

Step#8: Determining if the selected UPS is affordable.

Evaluation Criteria for Selecting an UPS-Part One


 

 

 

Example: Selecting an Uninterruptible Power Supply (UPS)

UPS System Ratings and Service Conditions

First: from IEC 60146-4

Second: according to American standards

Evaluation Criteria for Selecting an UPS-Part Two


 

The UPS sizing calculations steps:

Step#1: List All the UPS Loads

Step#2: List for Each Equipment/Load, the Voltage, Number of Phases, and Frequency

Step#3: List the KVA for Each Equipment/Load

Step#4: Determine The UPS Voltage, Number Of Phases, and Frequency.

Step#5: Segregate the Loads (Non-Motor Loads & Motor Loads)

Step#6: Determining Load Power Factor and KW Demand

Step#7: Determining Load Inrush Current/KVA.

Step#8: Determine Loads’ Sequence of Operation

Step#9: Apply the Derating Factors (If Any)

Step#10:  Calculate the Design UPS Load KVA

 

Stationary UPS Sizing Calculations – Part One


 

2- Rectifier/Charger Sizing Calculations

3- Inverter sizing calculations & Static Switch Sizing

4- The Battery sizing calculations

First: The Manufacturers’ methods, which include:

  • Method#1:Watts per cell method
  • Method#2:Watts per bank method
  • Method#3:Ampere per cell method

 

Stationary UPS Sizing Calculations – Part Two